Measurement of ambient concentrations of benzene, toluene and xylene in the vicinity of petrol service stations
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Ambient air monitoring in the vicinity of three petrol service stations was undertaken to determine concentrations of three volatile organic compounds; benzene, toluene and xylene (BTX). Quantitative measurements were undertaken using both diffusive and active sampling onto a solid adsorbent material followed by thermal desorption and a gas chromatography/mass spectrometry (GC/MS) determination. Mean weekly BTX concentrations are reported for forecourt pumps, forecourt perimeter, roadside and background sampling locations over a four-week monitoring campaign. Diffusive sampling was performed using Chromosorb 106 and Tenax, and a high degree o f correlation was observed between the BTX concentrations measured using the two adsorbent materials. The Wilcoxon Signed Rank Test revealed no significant difference in the measured benzene concentrations, while the toluene and xylene levels differed significantly using the two adsorbent materials. The observed difference in the measured concentrations may be attributed to the lack of validation of the diffusive sampling rates for Tenax at environmental levels for one-week sampling periods. The observed rank order of BTX concentrations at the sampling locations was; forecourt pumps area > forecourt perimeter > roadside > background locations where mean BTX concentrations were 7.3, 4.5, 2.2 and 0.9pg/m3, respectively. The BTX concentrations measured at the forecourt pumps area of all three stations were approximately 5 to 15 times higher than the corresponding background levels. Concentrations measured in the forecourt pumps and perimeter areas were shown to be dependant on the scale of the service station and prevailing meteorological conditions. It is suggested that the volume of traffic is the predominant source of BTX measured at the roadside sampling locations. Mean background concentrations of BTX were approximately lpg/m3. Ratios of measured BTX concentrations at forecourt pumps, forecourt perimeteT and roadside sampling locations were consistent with expected values. However, the BTX ratios at the background sampling location Station A seem to be influenced by an alternative source of xylene other than petrol vapour or vehicular emissions. BTX concentrations measured at forecourt service areas in the current study were considerably lower than previous studies where levels of up to 187, 325 and 142pg/m3, respectively for BTX have been reported. Background concentrations were also shown to be lower than previously reported results, however, in the current study the background locations were sub-urban as opposed to the urban site selected for previous studies. Mean benzene concentrations at the forecourt pumps area were approximately 50% higher than the proposed EU annual average limit value of 5pg/m3. The highest weekly toluene concentration recorded for the study was 29.7p.g/m3, which is significantly lower than the WHO guideline value of 260pg/m . Higher BTX concentrations were observed for the short four-hour active sampling period at Station C, the results however confirmed the rank order of sampling locations observed for the diffusive monitoring campaign.
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